Apart from the multiplying and integrating components the machine employs only relays and simple electronic circuits. Operation is based upon the storage of samples of the input signal on a number of condensers connected to the contact banks of 2 uniselectors, the wipers of which can be rotated at the same speed but with any desired angular separation equivalent to the required values of the correlation interval (Tau). An instrument was constructed and tested with an input approximating a sine wave. The amplitude was 30 v and the equivalent sampling rate was 12/c. The tests demonstrated that the principle is sound and that the instrument built provides the basis for a practical machine.